Monovalent cation stimulation of Ca2+ uptake by cardiac membrane vesicles

Sarcoplasmic Reticulum from Horse Gluteal Muscle Is Poised for Enhanced Calcium Transport

We have analyzed the enzymatic activity of the sarcoplasmic reticulum (SR) Ca²⁺-transporting ATPase (SERCA) from the horse gluteal muscle. Horses are bred for peak athletic performance yet exhibit a high incidence of exertional rhabdomyolysis, with elevated levels of cytosolic Ca²⁺ proposed as a correlative linkage. We recently reported an improved protocol for isolating SR vesicles from horse muscle; these horse SR vesicles contain an abundant level of SERCA and only trace-levels of sarcolipin (SLN), the inhibitory peptide subunit of SERCA in mammalian fast-twitch skeletal muscle. Here, we report that the in vitro Ca²⁺ transport rate of horse SR vesicles is 2.3 ± 0.7-fold greater than rabbit SR vesicles, which express close to equimolar levels of SERCA and SLN. This suggests that horse myofibers exhibit an enhanced SR Ca²⁺ transport rate and increased luminal Ca²⁺ stores in vivo. Using the densitometry of Coomassie-stained SDS-PAGE gels, we determined that horse SR vesicles express an abundant level of the luminal SR Ca²⁺ storage protein calsequestrin (CASQ), with a CASQ-to-SERCA ratio about double that in rabbit SR vesicles. Thus, we propose that SR Ca²⁺ cycling in horse myofibers is enhanced by a reduced SLN inhibition of SERCA and by an abundant expression of CASQ. Together, these results suggest that horse muscle contractility and susceptibility to exertional rhabdomyolysis are promoted by enhanced SR Ca²⁺ uptake and luminal Ca²⁺ storage.

Purification of sarcoplasmic reticulum vesicles from horse gluteal muscle

We have analyzed protein expression and enzyme activity of the sarcoplasmic reticulum Ca²⁺-transporting ATPase (SERCA) in horse gluteal muscle. Horses exhibit a high incidence of recurrent exertional rhabdomyolysis, with myosolic Ca²⁺ proposed, but yet to be established, as the underlying cause. To better assess Ca²⁺ regulatory mechanisms, we developed an improved protocol for isolating sarcoplasmic reticulum (SR) vesicles from horse skeletal muscle, based on mechanical homogenization and optimized parameters for differential centrifugation. Immunoblotting identified the peak subcellular fraction containing the SERCA1 protein (fast-twitch isoform). Gel analysis using the Stains-all dye demonstrated that calsequestrin (CASQ) and phospholipids are highly enriched in the SERCA-containing subcellular fraction isolated from horse gluteus. Immunoblotting also demonstrated that these horse SR vesicles show low content of glycogen phosphorylase (GP), which is likely an abundant contaminating protein of traditional horse SR preps. The maximal Ca²⁺-activated ATPase activity (V_max) of SERCA in horse SR vesicles isolated using this protocol is 5‒25-fold greater than previously-reported SERCA activity in SR preps from horse skeletal muscle. We propose that this new protocol for isolating SR vesicles will be useful for determining enzymatic parameters of horse SERCA with high fidelity, plus assessing regulatory effect of SERCA peptide subunit(s) expressed in horse muscle.

Nucleotide activation of the Ca-ATPase

We have used fluorescence spectroscopy, molecular modeling, and limited proteolysis to examine structural dynamics of the sarcoplasmic reticulum Ca-ATPase (SERCA). The Ca-ATPase in sarcoplasmic reticulum vesicles from fast twitch muscle (SERCA1a isoform) was selectively labeled with fluorescein isothiocyanate (FITC), a probe that specifically reacts with Lys-515 in the nucleotide-binding site. Conformation-specific proteolysis demonstrated that FITC labeling does not induce closure of the cytoplasmic headpiece, thereby assigning FITC-SERCA as a nucleotide-free enzyme. We used enzyme reverse mode to synthesize FITC monophosphate (FMP) on SERCA, producing a phosphorylated pseudosubstrate tethered to the nucleotide-binding site of a Ca(2+)-free enzyme (E2 state to prevent FMP hydrolysis). Conformation-specific proteolysis demonstrated that FMP formation induces SERCA headpiece closure similar to ATP binding, presumably due to the high energy phosphoryl group on the fluorescent probe (ATP·E2 analog). Subnanosecond-resolved detection of fluorescence lifetime, anisotropy, and quenching was used to characterize FMP-SERCA (ATP·E2 state) versus FITC-SERCA in Ca(2+)-free, Ca(2+)-bound, and actively cycling phosphoenzyme states (E2, E1, and EP). Time-resolved spectroscopy revealed that FMP-SERCA exhibits increased probe dynamics but decreased probe accessibility compared with FITC-SERCA, indicating that ATP exhibits enhanced dynamics within a closed cytoplasmic headpiece. Molecular modeling was used to calculate the solvent-accessible surface area of FITC and FMP bound to SERCA crystal structures, revealing a positive correlation of solvent-accessible surface area with quenching but not anisotropy. Thus, headpiece closure is coupled to substrate binding but not active site dynamics. We propose that dynamics in the nucleotide-binding site of SERCA is important for Ca(2+) binding (distal allostery) and phosphoenzyme formation (direct activation).

Synthesis and pharmacological characterization of functionalized 2-pyridones structurally related to the cardiotonic agent milrinone

A new class of cardiotonic agents characterized by a 2-pyridone structure was synthesized. Appropriate sym-2-dimethylaminomethylene-1,3-diones reacted with methylcyanoacetate to afford the desired compounds. These derivatives were evaluated for their ability in inducing cardiotonic response on guinea pig isolated myocardial preparations. Compound 8b increased atrial contractility to an extent which is significantly higher than that of milrinone, the parent drug used as a reference compound. The pharmacological characterization and the docking studies performed on 8b highlighted its selective mechanism of action via type 3 PDE (PDE3) inhibition.

Functional Co-expression of the canine cardiac Ca2+ pump and phospholamban in Spodoptera frugiperda (Sf21) cells reveals new insights on ATPase regulation

The utility of the baculovirus cell expression system for investigating Ca2+-ATPase and phospholamban regulatory interactions was examined. cDNA encoding the canine cardiac sarco(endo)plasmic Ca2+-ATPase pump (SERCA2a) was cloned for the first time and expressed in the presence and absence of phospholamban in Spodoptera frugiperda (Sf21) insect cells. The recombinant Ca2+ pump was produced in high yield, contributing 20% of the total membrane protein in Sf21 microsomes. At least 70% of the expressed pumps were active. Co-expression of wild-type, pentameric phospholamban with the Ca2+-ATPase decreased the apparent affinity of the ATPase for Ca2+, but had no effect on the maximum velocity of the enzyme, similar to phospholamban's action in cardiac sarcoplasmic reticulum vesicles. To investigate the importance of the oligomeric structure of phospholamban in ATPase regulation, SERCA2a was co-expressed with a monomeric mutant of phospholamban, in which leucine residue 37 was changed to alanine. Surprisingly, monomeric phospholamban suppressed SERCA2a Ca2+ affinity more strongly than did wild-type phospholamban, demonstrating that the pentamer is not essential for Ca2+ pump inhibition and that the monomer is the more active species. To test if phospholamban functions as a Ca2+ channel, Sf21 microsomes expressing either SERCA2a or SERCA2a plus phospholamban were actively loaded with Ca2+ and then assayed for unidirectional 45Ca2+ efflux. No evidence for a Ca2+ channel activity of phospholamban was obtained. We conclude that the phospholamban monomer is an important regulatory component inhibiting SERCA2a in cardiac sarcoplasmic reticulum membranes, and that the channel activity of phospholamban previously observed in planar bilayers is not involved in the mechanism of ATPase regulation.

Cardiac sarcoplasmic reticulum Ca2+ pump as a target for benzoquinones

1. Three benzoquinones (p-benzoquinone, 2,5-dimethyl-p-benzoquinone and tetramethyl-1,4-benzoquinone), differing for their electrophilicity, were tested on Ca2+ ATPase activity of cardiac sarcoplasmic reticulum membrane vesicles. 2. Only p-benzoquinone and 2,5-dimethyl-p-benzoquinone inhibited Ca2+ ATPase activity in a time-and concentration-dependent way, tetramethyl-1,4-benzoquinone being ineffective. 3. p-Benzoquinone and 2,5-dimethyl-p-benzoquinone decreased in a concentration-dependent way the total--SH groups of cardiac sarcoplasmic reticulum vesicles. 4. The results suggest that the most electrophilic benzoquinones interact directly with some critical--SH groups of Ca2+ pump inhibiting the enzymatic activity.

Purified, reconstituted cardiac Ca2+-ATPase is regulated by phospholamban but not by direct phosphorylation with Ca2+/calmodulin-dependent protein kinase

Regulation of calcium transport by sarcoplasmic reticulum provides increased cardiac contractility in response to beta-adrenergic stimulation. This is due to phosphorylation of phospholamban by cAMP-dependent protein kinase or by calcium/calmodulin-dependent protein kinase, which activates the calcium pump (Ca2+-ATPase). Recently, direct phosphorylation of Ca2+-ATPase by calcium/calmodulin-dependent protein kinase has been proposed to provide additional regulation. To investigate these effects in detail, we have purified Ca2+-ATPase from cardiac sarcoplasmic reticulum using affinity chromatography and reconstituted it with purified, recombinant phospholamban. The resulting proteoliposomes had high rates of calcium transport, which was tightly coupled to ATP hydrolysis (approximately 1.7 calcium ions transported per ATP molecule hydrolyzed). Co-reconstitution with phospholamban suppressed both calcium uptake and ATPase activities by approximately 50%, and this suppression was fully relieved by a phospholamban monoclonal antibody or by phosphorylation either with cAMP-dependent protein kinase or with calcium/calmodulin-dependent protein kinase. These effects were consistent with a change in the apparent calcium affinity of Ca2+-ATPase and not with a change in Vmax. Neither the purified, reconstituted cardiac Ca2+-ATPase nor the Ca2+-ATPase in longitudinal cardiac sarcoplasmic reticulum vesicles was a substrate for calcium/calmodulin-dependent protein kinase, and accordingly, we found no effect of calcium/calmodulin-dependent protein kinase phosphorylation on Vmax for calcium transport.

Calcium transport and calcium activated ATPase activity in microsomal vesicles of rat gastric mucosa

1. Microsomal and plasma membrane vesicles, isolated from rat gastric mucosa, were found to exhibit Ca(2+)-dependent ATPase activities of 14.1 +/- 1.4 and 7.8 +/- 1.1 mumol/mg/hr, respectively. The optimum conditions for the microsomal Ca(2+)-ATPase was pH 6-7, and required Mg2+, while divalent cation such as Cu2+, Zn2+, Fe2+, Ba2+ and Cd2+ had no significant effect. 2. As in the case of Ca2+, Mg(2+)-ATPase, the Ca2+ uptake activity of the microsomal membrane required Mg2+. Both processes were stimulated by submicro molar concentrations of Ca2+ and the apparent Km for Ca2+, Mg2+ ATPase and Ca2+ uptake activities were 0.06 microM and 0.02 microM, respectively. 3. Divalent cations Ba2+ and Fe2+, inhibited both microsomal activities, while Zn2+ and Cd2+ showed no effect on them. However, the monovalent cation K+ did not stimulate Ca2+, Mg(2+)-ATPase and Ca2+ uptake activities. 4. The Ca2+ pumping ATPase of rat gastric mucosal microsome cross-reacted with a monoclonal antibody (mAb-5F10) against the human erythrocyte Ca2+ pump. The apparent molecular weight of mucosal Ca2+ pump was 98 kDa. 5. Close relationship between the kinetic parameters of Ca2+, Mg(2+)-ATPase and Ca2+ uptake activities, and the cross reaction of 98 kDa protein of mucosal microsome with erythrocyte Ca2+ pump antibody, strongly suggest the expression of Ca2+ pump in rat gastric mucosa.

Differential molecular dynamics and transmembrane fluidity gradients in canine myocardial sarcolemma and sarcoplasmic reticulum

The molecular dynamics of highly purified preparations of canine myocardial sarcolemma (SL) and sarcoplasmic reticulum (SR) were quantified by electron spin resonance spectroscopy (ESR). Canine myocardial SL and SR have substantially different motional regimes in their membrane interiors as demonstrated by alterations in the relative peak height ratios, peak widths and peak splittings in ESR spectra of 16-doxylstearate incorporated into SL and SR. Quantification of the apparent order parameters (S) of 16-doxylstearate in SL and SR by analyses of ESR spectra demonstrated that the interior of the SL membrane was substantially more immobilized than the interior of the SR membrane (e.g. S = 0.168 +/- 0.002 for SL and S = 0.128 +/- 0.003 for SR). In contrast, only modest differences in membrane dynamics near the hydrophobic-hydrophilic interface were present in SL and SR as ascertained by ESR spectra of the probe 5-doxylstearate incorporated into these membranes. Myocardial sarcolemma contained heretofore unsuspected amounts of cholesterol (1.4 +/- 0.1 mumol cholesterol/mg protein) while sarcoplasmic reticulum contained only small amounts of cholesterol (0.17 +/- 0.06 mumol cholesterol/mg protein). Model systems employing binary mixtures of plasmenylcholine/cholesterol and phosphatidylcholine/cholesterol demonstrated that the observed alterations in molecular dynamics were due, in large part, to the differential cholesterol content in these two subcellular membrane compartments. Taken together, these results demonstrate that these two functionally distinct myocardial subcellular membranes have markedly disparate molecular dynamics and transmembrane fluidity gradients which may facilitate their performance of specific functional roles during excitation-contraction coupling in myocardium.

Interaction of alkaline metal ions with Ca(2+)-binding sites of Ca(2+)-ATPase of sarcoplasmic reticulum: 23Na-NMR studies

The analysis of the 23Na-NMR signal shape variations in the presence of vesicles of light sarcoplasmic reticulum (SR) shows the existence of sodium sites on the membranes with Kd values of about 10 mM. Other monovalent cations displace Na+ from SR fragments in a competitive manner according to the row K+ greater than Rb+ greater than Cs+ greater than Li+. Calcium ions also reduce Na+ binding, the Na+ desorption curve being of a two-stage nature, which, as suggested, indicates the existence of two types of Ca(2+)-sensitive Na+ binding sites (I and II). Sites of type I and II are modified by Ca2+ in submicromolar and millimolar concentrations, respectively. Analysis of sodium (calcium) desorption produced by calcium (sodium) allowed us to postulate the competition of these two cations for sites I and identity of these sites to high-affinity Ca(2+)-binding ones on the Ca(2+)-ATPase. Sites I weakly interact with Mg2+ (KappMg approximately 30 mM). Reciprocal effects of sodium and calcium on binding of each other to sites II cannot be described by a simple competition model, which indicates nonhomogeneity of these sites. A portion of sites I (approximately 70%) interacts with Mg2+ (KappMg = 3-4 mM). The pKa value of sites II is nearly 6.0. The number of sites II is three times greater than that of sites I. In addition, sites with intermediate affinity for Ca2+ were found with Kd values of 2-5 microM. These sites were revealed due to the reducing of the sites II affinity for Na+ upon Ca2+ binding to SR membranes. It can thus be concluded that in nonenergized SR there are binding sites for monovalent cations of at least three types: (1) sites I (which also bind Ca2+ at low concentrations), (2) magnesium-sensitive sites II and (3) magnesium-insensitive sites II.

Effects of corn oil addition to the diet on the energy metabolism of heart, liver and kidney of female rats

Female rats (SIV-50, Sprague Dawley) given a diet enriched with corn oil (10% and 25% addition to rat chow, w/w) for four weeks showed changes in the energy metabolism of heart, liver and kidneys, that is, changes in oxygen consumption and uncoupling of oxidative phosphorylation of heart, liver and kidney mitochondria, concomitant to a decrease in heart mitochondrial creatine kinase activity and an increase in heart mitochondrial creatine content. The high fat diet also affected calcium binding and Na/K-, Mg- and Ca-ATPases of a cardiac myocyte membrane fraction. Lipid feeding also led to increase (biphasic and transient) of phospholipids, triglycerides and free fatty acids in the three organs studied, but in no case to hypertrophy.

Characteristics of Ca2(+)-stimulated ATPase in rat heart sarcolemma in the presence of dithiothreitol and alamethicin

We have studied the activities of Ca2(+)-stimulated ATPase in rat heart sarcolemma upon modulating the redox state of membrane thiol groups with dithiothreitol (DTT). The suitability of alamethicin to unmask the latent activity of this enzyme was also investigated. The Ca2(+)-stimulated ATPase in sarcolemma exhibited two activation sites--one with low affinity (Km = 0.70 +/- 0.2 mM; Vmax = 10.0 +/- 2.2 mumol Pi/mg/h) and the other with high affinity (Km = 0.16 +/- 0.7 mM; Vmax = 4.6 +/- 0.8 mumol Pi/mg/h) for Mg2+ ATP. Alamethicin at a ratio of 1:1 with the sarcolemmal protein caused a 3-fold activation of Ca2(+)-stimulated ATPase without affecting its sensitivity to Ca2+ or Mg2+ ATP. Treatment of sarcolemma with deoxycholate or sodium dodecyl sulfate resulted in a total loss of the enzyme activity; high concentrations of alamethicin also showed a detergent-like action on the sarcolemmal vesicles. DTT at 5-10 mM concentrations caused a 4-5 fold activation of Ca2(+)-stimulated ATPase in sarcolemma and this effect was observed to be dependent on the concentration of Mg2+ ATP. DTT increased the affinity of the enzyme to Mg2+ ATP at the high affinit site and enhanced the Vmax at the low affinity site in addition to increasing the sensitivity of Ca2(+)-stimulated ATPase to Ca2+. DTT protected the Ca2(+)-stimulated ATPase against deterioration by detergents and restored the enzyme activity after treatment with N-ethylmaleimide. The mechanism of action of DTT on Ca2(+)-stimulated ATPase may involve the reduction of essential thiols at the active site of the enzyme or its interaction with specific DTT-dependent inhibitor protein. No changes in the sensitivity of sarcolemmal Ca2(+)-stimulated ATPase to orthovanadate was evident in the absence or presence of DTT and alamethicin. The results suggest the use of both DTT and alamethicin for the determination of Ca2(+)-stimulated ATPase activity in sarcolemmal preparations.

Calcium transport by sarcoplasmic reticulum of skeletal muscle is inhibited by antibodies against the 53-kilodalton glycoprotein of the sarcoplasmic reticulum membrane

The effects of an antiserum against the 53-kDa glycoprotein (GP-53) of the sarcoplasmic reticulum (SR) and of monoclonal antibodies against GP-53 on Ca2+ transport and ATP hydrolysis by SR of rabbit skeletal muscle have been investigated. Preincubation of SR with an antiserum against GP-53 resulted in decreased ATP-driven Ca2+ transport by the SR but had no effect on Ca2+-stimulated ATP hydrolysis. Preincubation of SR with preimmune serum had no significant effect on either Ca2+ transport or Ca2+-ATPase activity. The effect of anti-GP-53 serum was time and concentration dependent. Preincubation of SR with two monoclonal antibodies against GP-53 had no effect on Ca2+ transport or on Ca2+-stimulated ATP hydrolysis. However, preincubation of SR with either monoclonal antibody against GP-53 together with a monoclonal antibody against the Ca2+-ATPase (at levels which had little effect alone) resulted in markedly decreased rates of Ca2+ uptake and ATP hydrolysis. Preincubation of SR with anti-GP-53-serum or with monoclonal antibodies, under the same conditions that inhibited Ca2+ uptake, did not increase the passive permeability of the SR membrane to Ca2+, did not decrease the permeability of the SR to oxalate, and did not cause significant proteolysis of the Ca2+-ATPase. Our results are consistent with the interpretation that GP-53 may modulate the function of the Ca2+-ATPase of the SR membrane.

Effects of 2-methyl-1,4-naphthoquinone (menadione) on myocardial contractility and cardiac sarcoplasmic reticulum Ca-ATPase

(1) In electrically driven guinea-pig left atria, menadione (2-methyl-1,4-naphthoquinone) (1 to 20 mumol/l) and menadione sodium bisulfite (30 to 200 mumol/l) produced marked positive inotropic effects. Endogenously released catecholamines and histamine contributed to 80-85% of the effect, the residual 15-20% appearing as a direct effect. (2) In electrically driven guinea-pig ventricular strips, low micromolar concentrations of menadione (0.05 to 0.3 mumol/l) exerted a catecholamine-mediated small positive inotropic effect. (3) In both myocardial preparations, the increase in force of contraction was followed by a non-reversible rise of resting force. In its effects on cardiac contractility menadione resembled the thiol group blocking agent p-chloromercuribenzoate and H2O2. Pretreatment of atria with glutathione prevented the increase in resting force, while dithiothreitol only slightly delayed it. By contrast, the pretreatment with the NAD(P)H-quinone reductase (DT-diaphorase) inhibitor, dicumarol, markedly increased the rate of appearance of the toxic effect of menadione. (4) Among enzymatic and transport systems involved in the onset and control of cardiac contractility, sarcoplasmic reticulum Ca-ATPase was significantly inhibited by menadione after a long contact time. The inhibition was concentration-dependent and persistent, and was antagonized by addition of glutathione. (5) On the basis of these results, the increase in resting force caused by menadione appears to be related to an impairment of the thiol groups of proteins (Ca-ATPase), presumably caused by the drug per se.

Inhibition of cardiac sarcoplasmic reticulum Ca2+-ATPase activity by menadione

2-Methyl-1,4-naphthoquinone (menadione) inhibits Ca2+-ATPase activity of cardiac sarcoplasmic reticulum membrane vesicles in a time- and concentration-dependent way; after 60 min of preincubation an apparent Ki value of 33.5 microM was calculated. Inhibition is not reversible in that it persists even after the drug is removed and Ca2+-ATPase activity is assayed in a menadione-free medium. GSH (2 mM), but not DTT, is able to prevent and reverse the inhibition of Ca2+-ATPase by menadione. The relative importance of menadione metabolism in the inhibition of Ca2+-ATPase was studied in cell-free systems composed of vesicles and subcellular fractions containing metabolizing enzymes. Under these experimental conditions, 105,000g supernatants isolated from heart or liver that biotransform menadione through DT-diaphorase reduce the inhibition of Ca2+-ATPase activity determined by menadione. Also liver microsomes that biotransform menadione through NADPH-cytochrome P450 reductase decrease the inhibition by menadione. By contrast, cardiac microsomes that do not biotransform the drug do not influence the effect of menadione. These results indicate that, under the experimental conditions used for this study, menadione does not require metabolism to inhibit cardiac sarcoplasmic reticulum Ca2+-ATPase activity.

Effects of N-chlorobenzyl analogues of amiloride on myocardial contractility, Na-Ca-exchange carrier and other cardiac enzymatic activities

1. In electrically driven guinea pig left atria, micromolar concentrations (2 mumol/l to 80 mumol/l) of N-chlorobenzyl derivatives of amiloride (o-chlorobenzamil and 3',4'-dichlorobenzamil) produced quantitatively similar positive inotropic effects. Contracture developed with 3',4'-dichlorobenzamil. Endogenously released catecholamines contributed 30% to the positive inotropic effect of o-chlorobenzamil but did not contribute at all to the effect of 3',4'-dichlorobenzamil. When tested in the presence of the inhibitor of phosphodiesterase isobutylmethylxanthine, o-chlorobenzamil antagonized its positive inotropic effect, whereas 3',4'-dichlorobenzamil potentiated it. o-Chlorobenzamil also antagonized the positive inotropic effect of ouabain in that it shifted its concentration-effect curve to the right. Moreover, o-chlorobenzamil prevented the appearance of ouabain toxicity in terms of a rise in the resting force. 2. Also, in electrically driven guinea pig papillary muscle, micromolar concentrations (5 mumol/l to 30 mumol/l) of both N-chlorobenzyl derivatives of amiloride produced a positive inotropic effect. This effect was more marked with 3',4'-dichlorobenzamil than with o-chlorobenzamil and was associated for both compounds with lengthening of relaxation time. 3. o-Chlorobenzamil and 3',4'-dichlorobenzamil influenced, though not to the same extent, several systems involved in the onset and in the control of cardiac contractility. 3',4'-Dichlorobenzamil inhibited with the same potency Na-K-ATPase, sarcotubular Ca-ATPase, Na-Ca-exchange carrier, cAMP-dependent phosphodiesterase isolated from bovine heart and oxidative phosphorylation of mitochondria isolated from rat liver. Low micromolar concentrations of o-chlorobenzamil mainly inhibited Na-Ca-exchange carrier and cAMP-dependent phosphodiesterase.(ABSTRACT TRUNCATED AT 250 WORDS)

Ionic and substrate requirements of the high affinity calcium pumping ATPase in endoplasmic reticulum of pancreas

1. Calcium transport and ATPase activities were determined in microsomal vesicles from pancreatic tissue enriched in endoplasmic reticulum membranes. 2. Calcium transport and ATPase share the following properties: (i) magnesium was required with a K0.5 of 0.7 mM and maximal pumping ATPase activity at 5 mM Mg-ATP; (ii) at saturating magnesium concentrations, calcium increased ATP splitting activity up to three times with an apparent K0.5 close to 0.3 microM calcium; (iii) potassium stimulated the high calcium affinity Mg2+-dependent ATPase and calcium transport. 3. The properties of the calcium pumping system fulfil the cationic and substrate requirements from a physiological point of view.

Modulation of ATP-dependent calcium extrusion and Na+/Ca2+ exchange across rat cardiac sarcolemma by calcium antagonists

The calcium antagonists verapamil, bepridil, nifedipine and nimodipine inhibited ATP-dependent 45Ca2+ uptake in purified rat ventricular sarcolemma vesicles dose dependently. This inhibition was preceded by a slight stimulation in the case of the two dihydropyridines, but not with bepridil and verapamil. In contrast, Na+/Ca2+ exchange was only inhibited by verapamil and bepridil and not affected by the dihydropyridines. The steepness of the inhibition curves was significantly different for the two processes. No stereoselectivity was found with either process for inhibition by the verapamil enantiomers. Inhibition of the exchange was not due to a decrease of the exchange velocity but to a decrease in exchange capacity. Variation of the antagonist preincubation time did not modify the inhibition of the uptake. The results indicate that two different sites, located at the inner surface of the sarcolemma are involved in the modulation of the ATP-dependent uptake and the Na+/Ca2+ exchange. However, the possibility cannot be ruled out that inhibition of the exchange process is also mediated by an extracellularly located site.

The mechanism of calcium uptake by liver microsomes: effect of anions and ionophores

The mechanism of calcium uptake by liver microsomes was investigated using various anions and ionophores. Calcium uptake was shown to be specific to microsomes and unlikely to be due to contamination by plasma membranes by correlation of calcium uptake to the marker enzymes specific for these two fractions. Under the conditions employed, phosphates, sulfate, chloride, acetate, nitrate, thiocyanate, maleate, succinate and oxalate all stimulated calcium uptake by microsomes, but to different degrees. The greatest effect (4-6-fold) was observed with phosphate. On the contrary, phosphate is the only anion that stimulates the plasma membrane calcium uptake to any significant degree. Treatment of isolated microsomes with 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene (DIDS) resulted in inhibition of ATP- and anion-dependent calcium uptake. A lipid-permeable organic acid such as maleate retained its ability to promote calcium uptake in DIDS-treated microsomes. However, a lipophilic anion, such as nitrate, stimulated calcium uptake only in the presence of the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP). In addition, 2 microM valinomycin, when added in the absence or presence of 10 to 100 mM K+, had no stimulatory effect on calcium uptake. These results appear to be consistent with a model in which the active uptake of calcium into microsomes involves electroneutral Ca2+-nH+ exchange.

Effect of reduced oxygen intermediates on sarcolemmal muscarinic receptors from canine heart

The effect of oxygen free radicals generated by xanthine-xanthine oxidase system and hydrogen peroxide were investigated on cardiac muscarinic cholinergic receptors. We have used highly enriched sarcolemmal preparations isolated from canine myocardium. Exposure of the sarcolemma to oxygen free radicals by xanthine-xanthine oxidase system resulted in a significant (P less than 0.05) decrease of Bmax of (3H)-QNB (4.66 +/- 0.51 to 2.68 +/- 0.22 pmoles/mg protein). Addition of superoxide dismutase (SOD) and catalase (10 micrograms/ml) resulted in a significant reversal of Bmax value to 3.72 +/- 0.39 pmoles per mg protein (p less than 0.05). However, the affinity constants of dissociation (KD) were not altered appreciably with the exposure to oxygen free radicals with or without scavengers. Hydrogen peroxide significantly depressed 3H-QNB binding to the receptors in a dose-dependent manner in a concentration range between 4.41 mM -441 mM. This depression was completely inhibited by 10 micrograms/ml catalase. The study demonstrates that the oxygen free radical species are capable of disrupting (3H)-QNB binding to the cardiac muscarinic receptors.

Highly enriched, minimally disrupted plasma membrane vesicles from aortic myocytes grown in primary culture

A plasma membrane-enriched fraction (fraction 1B) has been obtained from rat aortic myocytes grown in primary culture. Plasma membrane markers, 5'-nucleotidase and ouabain-sensitive (Na+ + K+)-ATPase, are enriched 4.1- and 8.7-fold, respectively, in this fraction. Although endoplasmic reticulum marker NADPH-cytochrome c reductase is the most enriched in mitochondrial and heavy sucrose density gradient fractions, substantial enrichment of this marker is also observed in membrane fraction 1. This membrane preparation therefore contains a certain quantity of endoplasmic reticulum. Cytochrome c oxidase is de-enriched by a factor of 0.04 in fraction 1, indicating that it is essentially clear of mitochondrial contamination. Homogenization of aortic media-intima layers using a whole-tissue technique induces greater disruption of mitochondria and subsequent contamination of membrane fractions than does the procedure for cell disruption. Analysis of electrophoretic gels, vesicle density distribution and electron micrographs of enriched membrane fractions provide evidence that plasma membrane enriched from cultured myocytes is less traumatized than comparable fractions obtained from intact tissue. The potential value of such a highly enriched, minimally disrupted plasma membrane preparation is discussed.

Effect of internal and external K+ on Na+-Ca2+ exchange in dialyzed squid axons under voltage clamp conditions

The effect of external and internal K+ on Na+o-dependent Ca2+ efflux was studied in dialyzed squid axons under constant membrane potential. With axons clamped at their resting potentials, external K+ (up to 70 mM) has no effect on Na+-Ca2+ exchange. Removal of Ki+ causes a marked inhibition in the Na+o-dependent Ca2+ efflux component. Internal K+ activates the Na+-Ca2+ exchange with low affinity (K 1/2 = 90 mM). Activation by Ki+ is similar in the presence or in the absence of Na+i, thus ruling out a displacement of Na+i from its inhibitory site. Axons dialyzed with ATP also show a dependency of Ca2+ efflux on Ki+. The present results demonstrate that Ki+ is an important cofactor (partially required) for the proper functioning of the forward Na+-Ca2+ exchange.

The hemolytic effect of Salmonella typhi Ty 2 porins

Two outer membrane proteins of Salmonella typhi Ty 2 were extensively co-purified. According to their migration in dodecylsulfate/polyacrylamide gel electrophoresis and solubility characteristics, these proteins are homologous to the 35-kDa and 36-kDa porins found in Salmonella typhimurium. A porin homologous to the 34-kDa one has not been found in S. typhi Ty 2. A critical step in the purification of porins is heating at 100 degrees C in 2% sodium dodecyl sulfate before Sephadex gel filtration. The absence of detergent in aqueous suspensions enhances porin aggregation, these aggregations inducing human red cell lysis. Porins obtained by an alternative procedure consisting of heating at 60 degrees C instead of 100 degrees C were also hemolytic. Using nanomolar concentration of porins a strong influence of temperature on the hemolytic effect was observed. Porin-induced hemolysis was inhibited with anti-porin serum, as well as by a treatment with phenylglyoxal, which reacts with the arginine residues of proteins. The membrane-disrupting ability of porins aggregates might explain some pathogenic characteristics of gram-negative bacterial infections.

Dysfunction of the sarcoplasmic reticulum in polymyositis

The transport of calcium in vesicles of sarcoplasmic reticulum isolated from muscle specimens from 6 patients with early, active polymyositis and from 11 controls was examined. The time courses of calcium uptake and calcium-dependent ATPase activity were measured simultaneously. Calcium uptake by sarcoplasmic reticulum vesicles from patients with polymyositis was 50% less than that by vesicles from controls (P less than 0.001). In contrast, no difference in calcium-dependent ATPase activity was noted between vesicles from patients with polymyositis and controls. The demonstrated defect may be important in the pathogenesis of muscle weakness in polymyositis.

Characterization of calcium uptake into rough endoplasmic reticulum of rat pancreas

ATP-dependent Ca2+ uptake into isolated pancreatic acinar cells with permeabilized plasma membranes, as well as into isolated endoplasmic reticulum prepared from these cells, was measured using a Ca2+ -specific electrode and 45Ca2+. Endoplasmic reticulum was purified on an isopycnic Percoll gradient and characterized by marker enzyme distribution. When compared to the total homogenate, the typical marker for the rough endoplasmic reticulum RNA was enriched threefold and the typical marker for the plasma membrane Na+,K+(Mg2+)ATPase was decreased 20-fold. When different fractions of the Percoll gradient were compared, 45Ca2+ uptake correlated with the RNA content and not with the Na+,K+(Mg2+)ATPase activity. The characteristics of nonmitochondrial Ca2+ uptake into leaky isolated cells and 45Ca2+ uptake into isolated endoplasmic reticulum were very similar: Calcium uptake was maximal at 0.3 and 0.2 mmol/liter free Mg2+, at 1 and 1 mmol/liter ATP, at pH 6.0 and 6.5, and free Ca2+ concentration of 2 and 2 mumol/liter, respectively. Calcium uptake decreased at higher free Ca2+ concentration. 45Ca2+ uptake was dependent on monovalent cations (Rb+ greater than K+ greater than Na+ greater than Li+ greater than choline+) and different anions (Cl- greater than Br- greater than SO4(2-) greater than NO3- greater than I- greater than cyclamate- greater than SCN-) in both preparations. Twenty mmol/liter oxalate enhanced 45Ca2+ uptake in permeabilized cells 10-fold and in vesicles of endoplasmic reticulum, fivefold. Calcium oxalate precipitates in the endoplasmic reticulum of both preparations could be demonstrated by electron microscopy. The nonmitochondrial Ca2+ pool in permeabilized cells characterized in this study has been previously shown to regulate the cytosolic free Ca2+ concentration to 0.4 mumol/liter. Our results provide firm evidence that the endoplasmic reticulum plays an important role in the regulation of the cytosolic free Ca2+ concentration in pancreatic acinar cells.

Studies on the structure of yeast phosphofructokinase

In this paper, we describe an efficient procedure for the purification of yeast phosphofructokinase. This procedure eliminates any time delay and enables to obtain an enzyme with minimum proteolytic alterations. The molecular weights of the oligomeric enzyme and of its constitutive subunits were both evaluated by means of several independent methods. However, the accuracy of each measurement was not sufficient to discriminate between an hexameric and an octameric structure of the enzyme oligomer. On the other hand, crosslinking experiments demonstrated the octameric structure of yeast phosphofructokinase. Obviously, some methods of molecular weight determination have led to erroneous results. In particular, our experiments show that the reliability of molecular weight determinations performed by gel filtration of native proteins must be considered with caution.

Depression of calcium transport in sarcoplasmic reticulum from diabetic rats: lack of involvement by specific regulatory mediators

Cardiac sarcoplasmic reticulum (SR) ATP-dependent Ca2+-uptake was found to be depressed in 4 month streptozotocin-induced diabetic rats. Calmodulin, cAMP-dependent protein kinase and K+ stimulated Ca2+-uptake to similar degrees in SR from both control diabetic rats. Long chain acylcarnitine (7 microM) decreased Ca2+-transport in control rats by 46% but only 26% in diabetic animals. The data suggests that the depression in cardiac SR Ca2+-uptake activity in diabetic rats is non-specific in origin and not a result of alterations in regulation of SR function.

Ca-ATPase activity in bovine parathyroid cells

Ca-ATPase is thought to function as a calcium extrusion pump that may regulate cytosolic calcium concentration. Because the parathyroid gland is among the few tissues that are directly regulated by extracellular calcium and because cytosolic calcium may be a mediator of the effects of extracellular calcium on parathyroid hormone secretion, we have investigated the presence of this enzyme in homogenates of parathyroid cells. High performance liquid chromatography (HPLC) was used to quantify the formation of ADP from ATP following incubation of ATP with cellular homogenate in a buffer containing ethylenedioxy- (diethylenedinitrilo) tetra acetic acid (EGTA), ouabain, and calcium. Enzyme activity was calcium-dependent, with Ca-ATPase showing two Km (Ca) values, 31 and 853 nM. High affinity Ca-ATPase activity was reduced by the calmodulin inhibitor, trifluoperazine (TFP), with half-maximal inhibition occurring at 7 X 10(-5) M. Monovalent cations stimulated high affinity Ca-ATPase activity (K+ greater than Na+ greater than Rb+ greater than Li+) in the presence of calcium. Magnesium (0.8 mM) also stimulated cleavage of ATP. Sodium increased Ca-dependent ATPase activity by 82% but had no significant effect on Mg-stimulated activity. Furthermore, azide, an inhibitor of mitochondrial ATPase(s), had a significantly greater inhibitory effect on Mg-dependent than on Ca-dependent activity. In summary, a high affinity Ca-ATPase is present in bovine parathyroid cells which has a Km in the range of the cytosolic calcium concentration that is found in other cells. Ca-ATPase(s) may be of importance in regulating the cytosolic calcium concentration and, therefore, hormonal secretion in this cell type.

Effect of monovalent cations on Na+/Ca2+ exchange and ATP-dependent Ca2+ transport in synaptic plasma membranes

Two Ca2+ transport systems were investigated in plasma membrane vesicles isolated from sheep brain cortex synaptosomes by hypotonic lysis and partial purification. Synaptic plasma membrane vesicles loaded with Na+ (Na+i) accumulate Ca2+ in exchange for Na+, provided that a Na+ gradient (in leads to out) is present. Agents that dissipate the Na+ gradient (monensin) prevent the Na+/Ca2+ exchange completely. Ca2+ accumulated by Na+/Ca2+ exchange can be released by A 23187, indicating that Ca2+ is accumulated intravesicularly. In the absence of any Na+ gradient (K+i-loaded vesicles), the membrane vesicles also accumulate Ca2+ owing to ATP hydrolysis. Monovalent cations stimulate Na+/Ca2+ exchange as well as the ATP-dependent Ca2+ uptake activity. Taking the value for Na+/Ca2+ exchange in the presence of choline chloride (external cation) as reference, other monovalent cations in the external media have the following effects: K+ or NH4+ stimulates Na+/Ca2+ exchange; Li+ or Cs+ inhibits Na+/Ca2+ exchange. The ATP-dependent Ca2+ transport system is stimulated by increasing K+ concentrations in the external medium (Km for K+ is 15 mM). Replacing K+ by Na+ in the external medium inhibits the ATP-dependent Ca2+ uptake, and this effect is due more to the reduction of K+ than to the elevation of Na+. The results suggest that synaptic membrane vesicles isolated from sheep brain cortex synaptosomes possess mechanisms for Na+/Ca2+ exchange and ATP-dependent Ca2+ uptake, whose activity may be regulated by monovalent cations, specifically K+, at physiological concentrations.

Enrichment, solubilization, and partial characterization of digitonin-solubilized muscarinic receptors derived from canine ventricular myocardium

The subcellular distribution of cardiac muscarinic receptors was defined in canine ventricular myocardium, and receptors were solubilized from subcellular fractions enriched in muscarinic receptor content. The subcellular location of muscarinic receptors in cardiac tissue was determined by measurement of the distribution of [3H](+/-)quinuclidinyl benzilate-binding activity in particulate fractions isolated from canine ventricular myocardium. Based upon excellent correlation between [3H](+/-)quinuclidinyl benzilate binding and activity of the sarcolemmal Na+,K+-ATPase throughout the subcellular fractions, muscarinic receptors appeared to be localized to sarcolemma in canine ventricular myocardium. Therefore, membrane fractions enriched in sarcolemma were used as a source of cardiac muscarinic receptors for solubilization. Treatment of membrane vesicle fractions with digitonin (0.6%) resulted in solubilization of [3H](+/-)quinuclidinyl benzilate-binding activity with an extraction yield of 25-35%. Criteria of pharmacological specificity and stereospecificity established the identity of the solubilized binding activity of muscarinic receptors. Solubilization of muscarinic receptors was documented by demonstration of hydrodynamic behavior consistent with molecularly dispersed material. Upon glycerol gradient centrifugation, digitonin-solubilized muscarinic receptors from cardiac tissue sedimented with an apparent sedimentation coefficient of 9S. Pharmacological characterization of the digitonin-solubilized receptors revealed 8- to 39-fold reductions in affinities for muscarinic antagonists compared to the affinities exhibited by receptors in the membrane-bound state. Substantially greater reductions in agonist affinities (reduction of at least 700-fold for all agonists studied) suggested selective loss of ability of the digitonin-solubilized receptors to exhibit high affinity agonist interactions. In contrast to membrane-bound receptors, digitonin-solubilized receptors also demonstrated a loss of guanine nucleotide regulation, as well as steep agonist:radioligand competition curves with slope factors of 1.0, suggesting a homogeneous population of agonist-binding sites. Interpreted within the context of a model of state interconversion for membrane-bound receptors, the results suggested that either muscarinic receptors of a single state were selectively solubilized, or that solubilization induced conversion of all receptors to a single low affinity state, possibly by removal of constituents necessary for assumption of a high affinity agonist conformation.

Dependence of Na+-Ca2+ exchange and Ca2+-Ca2+ exchange on monovalent cations

Two mechanisms of passive Ca2+ transport, Na+-Ca2+ exchange and Ca2+-Ca2+ exchange, were studied using highly-purified dog heart sarcolemmal vesicles. About 80% of the Ca2+ accumulated by Na+-Ca2+ exchange or Ca2+-Ca2+ exchange could be released as free Ca2+, while up to 20% was probably bound. Na+-Ca2+ exchange was simultaneous, coupled countertransport of Na+ and Ca2+. The movement of anions during Na+-Ca2+ exchange did not limit the initial rate of Na+-Ca2+ exchange. Na+-Ca2+ exchange was electrogenic, with a reversal potential of about -105 mV. The apparent flux ratio of Na+-Ca2+ exchange was 4 Na+:1 Ca2+. Coupled cation countertransport by the Na+-Ca2+ exchange mechanism required a monovalent cation gradient with the following sequence of ion activation: Na+ much greater than Li+ greater than Cs+ greater than K+ greater than Rb+. In contrast to Na+-Ca2+ exchange, Ca2+-Ca2+ exchange did not require a monovalent cation gradient, but required the presence of Ca2+ plus a monovalent cation on both sides of the vesicle membrane. The sequence of ion activation of Ca2+-Ca2+ exchange was: K+ much greater than Rb+ greater than Na+ greater than Li+ greater than Cs+. Na+ inhibited Ca2+-Ca2+ exchange when Ca2+-Ca2+ exchange was supported by another monovalent cation. Both Na+-Ca2+ exchange and Ca2+-Ca2+ exchange were inhibited, but with different sensitivities, by external MgCl2, quinidine, or verapamil.

Assay of Na,K-ATPase in plasma membrane preparations: increasing the permeability of membrane vesicles using sodium dodecyl sulfate buffered with bovine serum albumin

Determination of maximal Na,K-ATPase activity in isolated plasma membranes is generally hampered by the vesicular nature of the preparation, limiting access of ATP and ions to one face or the other of the transmembrane protein. Detergents are often used to make the vesicles permeable to the substrates; however, the detergent/protein ratio is extremely critical for optimal activation. The use of bovine serum albumin as a detergent buffer is described. With this method the amount of membrane protein in the assay can be varied over a wide range, with full detergent activation. The method has been used for assay of Na,K-ATPase activity of membranes from dog kidney, rabbit brain, and electric organ of eel.